FIG. 1 illustrates tasks that have to be performed by the physical layer of telecommunications user equipment (UE) according to the European Telecommunications Standards Institute (ETSI) 3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE) standard.
When user equipment (UE) enters an LTE network, it has to perform cell search to detect the cell identifiers (IDs) of potential base stations that are referred to as enhanced node Bs (eNBs) in LTE terminology. Also it has to perform measurements on each detected cell and to report reference signal receive power (RSRP) and related Reference Signal Receive Quality (RSRQ). Once the UE has established a connection to an eNB, data reception is possible, and feedback information sent from the UE to the serving eNB helps to optimally utilize the bandwidth available to the eNB and to satisfy quality-of-service (QoS) requirements. Both feedback information as well as uplink payload data is sent via a “data transmission” block.
As shown in FIG. 1, a typical LTE UE implementation has to comprise computation resources for:                cell search (10): PSS (primary synchronization signal) and SSS (secondary synchronization signal) detection to determine cell IDs;        measurement (12): RSRP (reference signal receive power), RSRQ (reference signal receive quality);        data reception (14): PCFICH (physical control format indicator channel), PHICH (physical hybrid ARQ channel), PDCCH (physical downlink control channel), PDSCH (physical downlink shared channel), PMCH (physical downlink multicast channel);        feedback information computation (16): CQI (channel quality indicator), PMI (precoding matrix index), RI (rank indicator);        data transmission (18): PRACH (physical random access channel), PUCCH (physical uplink control channel), PUSCH (physical uplink shared channel).        
According to ETSI TS 136 214 V8.7.0, section 5.1.1, “Reference signal received power (RSRP) is defined as the linear average over the power contributions (in Watts) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth. For RSRP determination the cell-specific reference signals R0 according to ETSI TS 136 211 V8.9.0, section 6.10.1 shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP.” ETSI TS 136 211 V8.9.0, section 6.10.1 defines cell-specific reference symbols as shown in FIG. 2. An eNB may utilize 1, 2, or 4 transmit (Tx) antennas, each sending a certain pattern of cell-specific reference symbols. R0 and R1 represent the mapping of reference symbols in a resource block for a first and second Tx antenna, respectively, such as defined by the cited standard for an eNB using two Tx antennas.
Each column in the time-frequency plane corresponds to subcarriers of one OFDM symbol. Each of the two rectangular schemes corresponds to one resource block,i.e.12 subcarriers in frequency direction, and to 1 subframe, i.e. 2 slots, 1 millisecond in time direction, for the case of a normal cyclic prefix duration of 14 OFDM symbols. The shaded squares represent OFDM subcarriers carrying the cell-specific reference symbols, and the blank squares correspond to OFDM subcarriers carrying other kinds of symbols, mostly data symbols. The pattern shown in FIG. 2 is repeated in frequency direction to cover the actual transmission bandwidth. The total number of OFDM subcarriers in frequency direction may be up to 1200, for a system operating at 20 MHz bandwidth. This corresponds to 200 reference symbols in frequency direction, per Tx antenna. It is important to note that, within one OFDM symbol, these reference symbols are spaced equidistantly in frequency direction, respectively, for subcarriers below and above DC.
Since the DC carrier is left empty in LTE downlink signals (see ETSI TS 136 211 V8.9.0, section 6.12), this unused, inserted, DC carrier imposes a discontinuity in the otherwise equidistant reference symbol grid. The pattern illustrated in FIG. 2 is shifted in frequency direction in relation to the cell identifier (ID) associated with an eNB. Also different reference symbol modulation phase sequences are applied in dependence of the cell ID.
The frequency re-use factor in an LTE network is one, i.e., all eNBs operate at the same center frequency. The LTE standard does not foresee exact time synchronization between eNBs. Thus, a UE will receive superimposed signals from multiple base stations, with the respective frame structures mutually shifted in time and with respective reference symbol patterns as shown in FIG. 2 shifted to various positions in frequency direction.
Upon UE request, an eNB may permit a time pattern of so-called measurement gaps, which are intervals where a UE will never have to receive any data, which are left to the UE to be used for purposes like RSRP measurement. Configuration options are either 6 ms gap every 40 ms or 6 ms gap every 80 ms. During the gap duration, a UE implementation may utilize resources which are normally reserved for data reception, for other purposes, such as RSRP measurement. However, under certain circumstances an eNB may deny provision of measurement gaps. In a UE implementation this means that RSRP computation has to be performed concurrently with normal data reception. This means that resources from the data reception path will not be freed, and extra resources will be required for RSRP.
Intra-frequency measurement requirements are defined in ETSI TS 136133 V8.10.0, section 8.1.2.2. In particular, “when no measurement gaps are activated, the UE shall be capable of performing RSRP and RSRQ measurements for 8 identified intra-frequency cells, and the UE physical layer shall be capable of reporting measurements to higher layers with the measurement period of 200 ms”. These 8 intra-frequency cells will typically be received at different synchronization times. To avoid inter-symbol interference (ISI), each synchronization time corresponds to a specific sample extraction and FFT. Thus, when the goal is to report one RSRP measurement per subframe (i.e., 8 cells over a period of 8 subframes), 4 extra FFT operations are needed per subframe—to extract reference symbols from 4 OFDM symbols—in addition to the 14 FFT operations needed for data reception of this subframe.
FIG. 3 shows a block diagram of a straightforward way to implement RSRP measurement functionality for one receive antenna. Basically, blocks 20 to 28 compute a set of channel estimates across the frequency band, and the results are squared and summed in block 30 in order to obtain a single RSRP value.
When using the straightforward implementation depicted in FIG. 3 for RSRP computation, only up to one third of the OFDM subcarriers are actually used after FFT, namely those which actually carry reference symbols, which is quite inefficient.
Due to LTE requirements, under continuous operation conditions without measurement gaps, a UE implementation will have to utilize dedicated resources for data reception (typically together with feedback information computation), and dedicated resources for measurement.
A problem in the described RSRP measurement is that received signals from eNBs of other cells arrive at various different time synchronizations at the UE, compared to the signal arriving from the serving cell eNB. The signal as received in the user equipment is a superposition of the signals received from all eNBs in the surrounding. To perform measurements on multiple cells, the UE has to process the received signal multiple times, which requires memory for buffering purposes as well as extra processing resources. RSRP measurement results from up to 8 neighboring eNBs must be reported simultaneously with data reception. To save energy, the complexity required for RSRP measurement must be kept low.
An object of the invention, therefore, is to provide a low complexity method for RSRP measurement in an LTE UE receiver.